Impulse drying occurs when a wet paper web passes through the press nip of a pair of rolls in which one of the rolls is heated to a high temperature. A steam layer adjacent to the heated surface grows and displaces water from the sheet in a more efficient manner than conventional evaporative drying. It is projected that wide commercialization of impulse drying would result in very large industry wide energy savings.
Impulse drying is described in U.S. Pat. No. 4,324,613 to Wahren. Impulse drying is drying by means of heating one of a pair of rolls to a high temperature prior to passing a paper web between a pair of rolls. In the method of the Wahren patent, the surface of one of the rolls is heated to a high temperature by an external heat source immediately prior to passing the paper web between the heated roll and another roll. The Wahren patent describes the use of solid rolls having at least a surface layer having high thermal conductivity and high thermal diffusivity, such as copper or cast iron, for use as the heated roll.
The Wahren patent teaches that, in normal cases, a major part of the drying must take place in the press nip and final drying takes place after the nip. It is concluded the conductivity of the material of which the heating roll is made must be high so as not to dry at roll surface temperatures higher than necessary. A high conductivity means that the heat can be conducted to a greater depth in the roll and even extracted from a greater depth, which in itself means that a lower roll temperature can be used.
According to the Wahren patent, the choice of material is limited by the risk of thermal fatigue and, in this respect, at least the surface layer of the roll should be made of a material for which the quantity ##EQU1## has a high value, desirably at least 0.6.times.10.sup.6, where .sigma..mu. is the fatigue strength, .upsilon. is Poisson's ratio, .rho. is the density, c is the specific thermal capacity, .lambda. is the thermal conductivity, E is the modulus of elasticity, and a.sub.c is the coefficient of thermal expansion for the material. Copper alloys have the highest values, approximately 1.3.times.10.sup.6. However, they have rather poor resistance to wear and are not suitable for doctoring. Other suitable materials are duralumin (0.7.times.10.sup.6), cast iron (0.67.times.10.sup.6 -0.85.times.10.sup.6), steel (0.8.times.10.sup.6) and nickel (approximately 0.8.times.10.sup.6 -0.9.times.10.sup.6).
Thus, the Wahren patent teaches the use of high conductivity surfaces, such as metal surfaces on the heated roll used in impulse drying. The Wahren patent does not teach or recognize the desirability of using heated roll surfaces made from a material with a low value of the quantity K=.sqroot..rho.c.lambda. such as are used in the heated roll of the present invention.
In addition to the impact on energy consumption, impulse drying also has an effect on paper sheet structure and properties. Surface fiber conformability and interfiber bonding are enhanced by transient contact with the hot surface of the roll. As the impulse drying process is usually terminated before the sheet is completely dried, internal flash evaporation results in a distinctive density profile through the sheet that is characterized by dense outer layers and a bulky midlayer. For many paper grades, this translates into improved physical properties. The persistent problem with the use of impulse drying, however, is that flash evaporation can result in delamination of the paper sheet. This is particularly a problem with heavy weight grades of paper. This has been a major constraint as to the commercialization of impulse drying.
It has been reported, Crouse, J. W., et al., "Delamination: A Stumbling Block to Implementation of Impulse Drying Technology for Liner Board", Tappi Engineering Conference, Atlanta, Ga., Sep. 13, 1989, that various degrees of delamination were experienced with liner board dried at press roll surface temperatures above 150.degree. C. (300.degree. F.). When delamination was avoided by operating at the lowest limit, water removal efficiencies were not significantly different than those obtained by conventional drying. It is concluded in this report that to realize the potential of impulse drying, it would be necessary to alleviate delamination.
In laboratory scale simulations, Lavery, H. P. "High Intensity Drying Processes-Impulse Drying Report", Three DOE/CE/407383-T3, February, 1988, it was found that increased pulp refining encouraged delamination and it was postulated that very thick or highly refined sheets exhibit greater resistance to the flow of vapor than thin or coarse paper webs. Hence, if the flow resistance of the web became so large that high pressure steam could not escape, the sheet may not be strong enough to sustain the pressurized vapor and delamination would occur.
The effect of hot surface materials on delamination has been investigated, Santkuyl, R. J., "The Effect of Hot Surface Material on Delamination in Impulse Drying", Master's Program, Institute of Paper Science and Technology, March, 1989. Using an electrohydraulic impulse drying simulator, carbon steel, aluminum and sintered porous stainless steel platens were tested in terms of their ability to dewater and suppress delamination. A felt back-up pad was used in the simulations. It was observed that a difference in K value between steel (K value of 15,000 w.sqroot.s/m.sup.2 c) and aluminum (K value of 22,000 w.sqroot.s/m.sup.2 c) had no affect on dewatering capacity or the propensity for paper sheets to delaminate. Porous stainless steel (K value of 3000 w.sqroot.s/m.sup.2 c) platens provided completely suppressed delamination, although also providing considerable lower dewatering capacity. For porous materials, such as sintered porous stainless steel, a mass balance on the paper sheet showed that a large fraction of the water was removed as vapor and a much smaller fraction was displaced as liquid water into the backup felt. It was concluded that the porous platens do not operate by an impulse drying mechanism. Instead, steam formation and venting at the hot platen-vapor interface augmented by hot pressing were considered to be responsible for water removal. As a result of venting, measured temperatures within the vapor sheets never exceeded 100.degree. C. (212.degree. F.) and flash evaporation could not occur.
U.S. Pat. No. 3,296,710 to Krikorian is directed to the use of a porous absorbent layer on a roll to take up the water from the web. The water which is taken up in the pores of the porous roll is later evaporated by means of heating the porous layer. The use of a porous material is substantially different than the use of a solid material. The Krikorian patent is not related to the use of impulse drying. A porous material is not suitable for use as a roll for impulse drying since the porous material absorbs the moisture from paper in the nip of the rolls and such moisture is subsequently evaporated from the pores of the porous material.
Accordingly, it is a principal object of the present invention to provide a roll surface material which is suitable for use in impulse drying over a broad range of temperatures and nip residence times but wherein delamination of the paper web is prevented.
It is another object of the present invention to provide a roll surface material that can be heated for impulse drying and can attain efficiencies comparable to that of solid cast iron, copper or steel rolls but which do not result in delamination of the paper web under high energy transfer conditions.